Cumming Cockburn LimitedWater Budget Analysis on a Watershed Basis 2-1

2. Hydrologic Cycle andConcepts

2.1 Description of Hydrologic Cycle

The demand for water is continuous. In order to appropriate the availablewater, one should be familiar with elements that dictate the movements ofwater. For simplicity, where does the water come from? When is water inmotion, and is it in liquid or vapour form? Are there factors that intercept,trap and remove water by some means? Is the water stored duringmovement? How much is withdrawn by users? Since the hydrologic cycledescribes the process of water movement it is essential to thoroughlyunderstand the concepts of hydrology which provides a basis for theunderlying principles of a water budget. This section describes thehydrologic cycle and its components, as one would observe in thewatershed.

The hydrologic cycle describes the processes of motion, loss and rechargeof the earth’s water. The processes can be visualized as shown in Figure2.1. The cycle may be divided into the following principle components orphases:

a) Precipitationb) Interceptionc) Evaporation / Evapotranspirationd) Infiltratione) Percolationf) Runoff (Surface and Groundwater)g) Storage (Surface and Groundwater)h) Water Use and Demand

At any point in each of these phases, water may be in the state oftransportation, a state of temporary or long-term storage, or in a state ofchange between solid, liquid and gaseous flow.

Cumming Cockburn LimitedWater Budget Analysis on a Watershed Basis 2-2

Figure 2.1: Hydrologic Cycle - Visual Definition of MainHydrologic Components (after EPA, 1998)

Any exposed surface may be considered as a unit area on which thehydrologic cycle operates. For example:

• an isolated tree;

• the roof of a building;

• the drainage basin of a river system or any of its tributaries;

• a lake;

• an ocean;

• a country;

• a continent; and

• the earth as a whole.

A water balance is based upon the above noted factors and, is expressed asa mathematical equation showing the relationship among the varioushydrologic components. The equation is as follows:

Cumming Cockburn LimitedWater Budget Analysis on a Watershed Basis 2-3

P - E -T - Int - Ds + Qin + Gin - Qout - Gout ± ∆S = 0

where:P = Precipitation;E = Evaporation;T = Transpiration;Int = Interception;Ds = Detention Storage;Qin = Surface Inflow;Qout = Surface Outflow;Gin = Groundwater Inflow;Gout = Groundwater Outflow; and

∆S = Changes in Storage.

While precipitation generates the main input to the cycle, evaporation andtranspiration constitute the main losses of water from the groundwater andsurface water systems to the atmosphere. A schematic representation ofthe hydrologic cycle is shown on Figure 2.21.

From a global perspective, the world’s supply of water is stored mainly inoceans (97.2%), while 2.15% is frozen water (ice caps) and 0.65% is freshwater on land. Of this fresh water component, only 2.5% makes upquantities in air, soil, lakes and rivers, while the remainder is groundwater,most of which is unavailable for human use (Vander Leeden et al, 1990).

In a watershed, about one half to one third of precipitation enters the earth,while some of the available water remains at the surface. The quantity ofprecipitation that is available for runoff is a function of the topography, thesoil infiltration and the holding capacity of the surface. The mass balanceexpresses the relationship of rainfall excess or runoff, R, to be equal to theproduct of the precipitation, P, times the runoff coefficient, C, (R = CP),

where the storage is ∆S = (1-C) P. This runoff may be routed to streams,sewers, detention ponds or reservoirs. Typical uses of the stored water arerecreation, dilution capacity, water supply, irrigation, hydroelectric power,flood control, low flow augmentation, navigation, and natural resourcesenhancement.

1 A complete list of definitions and terms is provided in the List of Symbols, Abbreviations and

Glossary (Appendix A).

Cumming Cockburn LimitedWater Budget Analysis on a Watershed Basis 2-4

Figure 2.3: Canada Water Uses

Figure 2.2: A System Representation of the Hydrologic Cycle*

In Canada, major water uses are broken down as follows. Thermal andhydroelectric power generation use about 60%, manufacturing 20%,agriculture 9% and municipal about 11% (Environment Canada, 1992,Freshwater Series A-6) as shown in Figure 2.3.

Figure 2.3: Canada – Water Uses

ET P ET P ET P E

INT

R SR

I IF IF

PER BF

Gw

ATMOSPHERIC

VEGETATION

LAND SURFACE

SOIL MEDIA

AQUIFER

OCEAN

RIVERS

STREAMS

WETLANDS

LAKES

Legend *

P = PrecipitationET = EvapotranspirationIF = InterflowSR = Surface RunoffINT = Interception by

VegetationI = Infiltration from

Land Surface toSoil Media

PER = Percolation fromSoil Media toGroundwater(recharge)

BF = Base Flowprovidedby Groundwater toRivers and Streams

Gw = Groundwater Underflow